Change namespace from leveldb to rocksdb

Summary:
Change namespace from leveldb to rocksdb. This allows a single
application to link in open-source leveldb code as well as
rocksdb code into the same process.

Test Plan: compile rocksdb

Reviewers: emayanke

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13287

util/string_util.h

@@ -3,7 +3,7 @@
 #ifndef STORAGE_LEVELDB_UTIL_STRINGUTIL_H_
 #define STORAGE_LEVELDB_UTIL_STRINGUTIL_H_
 
-namespace leveldb {
+namespace rocksdb {
 
 extern std::vector<std::string> stringSplit(std::string arg, char delim);